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7.2.3.2 Pumping high gas loads by means of turbomolecular pumps

Turbopumps are subject to high stresses under high gas loads. Gas friction heats up the
rotors. The maximum gas loads are limited by the permissible rotor temperature of 120 °C.
Because the friction loss is proportional to the square of the peripheral speed, it is necessary
to reduce the RPM of pumps that operate under high gas loads. This means that higher gas
loads are attained at the expense of pumping speed, and in particular, at the expense of the
compression ratio; this is not a major disadvantage for these kinds of pumps, as they are not
used for generating high vacuum. Pumping heavy noble gases is particularly critical. Due to
their high atomic weights, heavy noble gases generate large quantities of heat when they
strike the rotor. As a result of their low specific thermal capacity, however, they can transfer
only little heat to the stator or to the housing, which results in high rotor temperatures. The
maximum gas loads for these gases are therefore relatively low.

When operating with process gases, the turbopump performs two important functions:

Fast evacuation of the process chamber to a low pressure(clean initial conditions)

Maintaining the desired pressure at a constant level during the vacuum process
(coating, etching, etc.)

Gas throughput Q and working pressure pa during a process are typically specified, and thus
the volume S = the process chamber, as well.

The turbopump will be selected on the basis of the required gas throughput. The maximum
permissible gas throughputs for various gases are specified for the respective pumps in the
catalog, with the throughput curves of turbopumps and backing pumps being used in this
connection (Figure 7.4). The throughput must be the same for both pumps, because the
same gas flow will pass through both pumps successively: Sv = .

The following rule of thumb applies for the backing pump: If the maximum gas throughput of
the turbopump is attained, the pumping speed of the backing pump must be selected high
enough so that only one half of the critical backing pressure will be utilized.

The volume flow rate at the process chamber is throttled to the required level by means
of either RPM or a regulating valve. It is frequently not possible to employ regulation as a
function of RPM, as it takes too long to set the desired pressure via RPM.

Example:

Let us consider a system in accordance with Figure 7.5.

Q

=

20 mbar · l/s gas throughput

pa

=

0.05 mbar process pressure

This results in a volume flow rate S of 400 l/s. We select a HiPace 2300 as the turbopump (2)
and a Uno 120 as the backing pump (3). With this backing pump, we can attain a backing
vacuum pressure of 0.8 mbar at a gas throughput of 20 mbar · 1/s, i.e. a little less than one
half of the critical backing pressure of 1.8 bar.

The process gas is admitted to the chamber (1) via a gas flow regulator (5). The butterfly
valve (4) that is controlled by pressure pa throttles the pumping speed of the turbopump (2).
After the conclusion of the process step, the gas supply is shut off and the control valve
opens completely to cleanly evacuate the chamber again. As this is happening, a new work
piece is loaded into the process chamber. Further information relating to pumping high
gas loads as well as corrosive and abrasive substances is provided in Section 2.7.3.